Polymers made by metallocene catalysts are gaining an increasing share of the worldwide plastics market. Many of these industrial catalysts are Kaminsky type ansa-metallocenes derived from 1. ##STR1## These catalysts require an anionic co-catalyst (or a catalyst activator), typically perfluorophenylborates or methyl aluminoxane (MAO), to generate the catalytically active cationic fourteen electron d.sup.0 species.
Of the two classes of co-catalysts, MAO-based systems predominate in industrial applications. Because MAO must be present in large molar excess (200-2000 times that of the metallocene catalyst), its presence accounts for over 50% of the cost of the catalytic system. In addition to the increased expense, little is known about MAO's structure or role in the polymerization reaction. Moreover, because MAO may also provide an additional route for chain termination (via chain transfer to the trimethyl aluminum), this complicates the design of more selective catalysts.
As a result, there have been many attempts to modify the Group IV metallocene based catalytic system to eliminate the need for MAO. Although perfluorophenylborates (i.e., perfluorinated tetra-alkyl borates) may be substituted as the co-catalyst, their thermal instability and extreme sensitivity to oxygen make them impractical for large scale industrial applications.
An attractive alternative is to design a single component Ziegler Natta catalyst that does not require any co-catalyst. One such attempt is Bercaw's series of iso-electronic Group III neutral metallocenes such as 2 (Burger, et al., Am. Chem. Soc. 112:1566 (1990)). Although Bercaw's catalysts have been enormously useful in elucidating the basic processes involved in olefin polymerization, they are of limited use in industrial applications because of their low polymerization activity. ##STR2##
Another approach is to incorporate a counter anion for a Group IV catalyst directly into the ligand. In one attempt, Jordan, et al. (Crowther, et al., J. Am. Chem. Soc. 113:1455 (1991)) prepared catalyst in which one cyclopentadienyl (Cp) ligand is replaced by a dianionic dicarbolide ligand. As a neutral d.sup.0 species, the resulting complex was expected to be capable of .alpha.-olefin polymerization. Unfortunately, these complexes readily undergo .beta.-elimination, have low activity, and only oligomerize polypropylene. The polarization of the dicarbolide-zirconium bond is believed to result in a more neutral metal center which makes the initial binding of the propylene less exothermic and the insertion into the Zr--R bond more disfavored since the Zr orbital would be less d-like. An attempt using a trimethylene methane ligand instead of the dicarbolide ligand also produced similar results (Rodriquez G. & Bazan G. C., J. Am. Chem. Soc. 113:1455 (1991)).
Consequently, a need exists for a single component Ziegler-Natta catalysts that is comparable to Kaminsky type ansa-metallocenes but without the need for a co-catalyst like MAO.